The long-term objectives of this proposal are to elucidate physiologic mechanisms that underlie the Sudden Infant Death Syndrome, and to develop age-appropriate, non-invasive tests that will identify infants who are at the greatest risk for SIDS. These tests focus on assessments of peripheral and central mechanisms involved in the integrated control of cardiac, blood pressure, and respiratory function. Subjects are the fetuses and infants of mothers from a low-socioeconomic status population in New York City and from a rural Native American population on the Pine Ridge Reservation in South Dakota, which is at unusually high risk for SIDS. The proposed studies begin with measurements made during late gestation. Assessments then continue through infancy to document the physiologic changes that coincide with the period of maximal risk for SIDS. These postnatal studies incorporate measurements made during sleep under basal conditions as well as during head-up and head-down tilting. The principal dependent variables are heart rate and several indices of heart rate variability, respiratory rate and variability, body temperature, and electrocortical activity and beat-to-beat blood pressure. Five experiments will address the following aims; Patterns of developmental change in cardiorespiratory activity measured from the late fetal period through infancy will distinguish groups of infants at increased risk for SIDS How postnatal development of heart rate, blood pressure and respiratory responses to the challenge of head-up and head-down tilting will be altered in groups of infants at increased risk for SIDS Physiologic responses to tilt are dependent upon sleep state and this dependency may change with age Blood pressure decreases to head-up tilt are larger in the prone sleeping position Head-up tilting will induce cortical activation which will be diminished during the period of greatest vulnerability to SIDS Respiratory and heart rate responses to tilt will be diminished in the morning hours and Responses to tilt will vary with time after feeding.